The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 120 nm. EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features, for example, sub-32 nm features, in substrates such as silicon wafers. These systems must be highly reliable and provide cost effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site.
The line-emitting element may be in pure form or alloy form, for example, an alloy that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam to a desired irradiation site simultaneously for plasma initiation presents certain timing and control problems, as it is necessary to hit the target properly in order to obtain a good plasma, and thus good EUV light.
One such problem involves the fact that there is generally a focusing lens that focuses the laser beam on the irradiation site. (There may also be other lenses between the laser source and the irradiation site, but only the final lens will directly focus the beam on the irradiation site.) It is desirable that the focal spot, or “waist,” of the focusing lens coincide with the irradiation site at which the target material is located so that the maximum effect of the laser energy may be obtained in forming the plasma. The terms focal “spot” and “waist” are used herein, rather than focal “point,” as physical lenses actually focus to a narrowest spot of measurable width, i.e., the focal spot or waist, rather than to an actual point as might be seen with a theoretical, mathematically perfect lens.
The focusing lens has a nominal focal length (the center of the focal spot) of a particular distance at a given temperature. Thus, in the absence of any other effect, the focal spot of the lens will produce the maximum intensity of the laser at a particular point of the laser path corresponding to the nominal focal length. It is well known, however, that the lens absorbs energy from the laser beam as the beam passes through the lens, and the lens will thus be expected to be subject to thermal effects which may change its focal length.
If the thermal load on the lens is constant, for example, if the laser is on continuously, then the lens will arrive at a steady state of this thermal effect in some period of time, typically on the order of a few minutes. The focal length of the lens under this steady state thermal load may be readily determined, and the lens may be placed such that the focal spot of the lens is located at the irradiation site when the lens is under thermal load, rather than when the lens is not under such load.
However, if the laser is turned on and off in periods of less than the time necessary for the lens to reach a steady state thermal load, but long enough to create some thermal load on the lens, then the focal spot may move slightly depending upon the particular amount of thermal load on the lens at any given moment.
There are at least two ways in which the laser may be turned on and off. First, in EUV systems, as in many integrated circuit production systems, there is generally a container called a “boat” which holds the wafers that are to be irradiated by the EUV beam; when the boat is changed to place a new set of wafers in the EUV beam path, the laser is typically turned off and no EUV light is produced during the period in which one boat is removed and the next inserted. This may generally take up to a minute or so, after which the laser is then turned back on, thus creating transient thermal effects both when the laser is turned off and when it is turned back on.
In addition, newer systems use laser pulses, and allow the user to set the conditions of the pulsing, and thus the production of EUV light. In one example, a burst of pulses for irradiating an exposure field on a wafer may include 20,000 pulses of 30 ns each, at a pulse repetition rate of 40 KHz, so that the total burst lasts for 0.5 second. In between bursts, the scanner holding the wafer re-aligns the wafer to allow for the irradiation of a different exposure field; this realignment may take, for example, 0.1 second.
The duty cycle is considered to be the percent of time that the light source, i.e., the laser, is operating at the specified pulse repletion rate. It is generally expected that a change in the duty cycle of more than about 20 percent will result in thermal transients in the lens, and that these transients may take several minutes to stabilize.
The amount of change in the focal length of the lens will vary with each particular lens and may not seem great; for example, a lens with a nominal focal length of 300 mm may vary by approximately 1 mm in either direction, i.e. from 299 to 301 mm, and possibly less than that. However, in comparison with a typical target size of 30 microns, this movement of the focal spot may be enough to reduce the coupling between the laser beam waist and the target and thus create problems in production of the plasma.
Prior attempts to compensate for thermal effects in EUV systems have concerned the focal spot of the EUV beam and the resulting exposure of the scanner, rather than the focal spot of the laser focusing lens. These are significantly different problems. The scanner is not part of the EUV light production, and thus any change in the focal spot of the EUV beam does not change the power produced by the EUV source, but only the location of maximum intensity of the EUV beam. Thus, in attempting to compensate for such a change in the focal length of the EUV beam it is sufficient to merely calculate the power received at the scanner over time, since whether the scanner is located at the focal spot or not does not change the power produced by the EUV source; if the scanner is not at the optimal focal spot, compensation for the decreased power of the EUV beam may generally be provided by lengthening the exposure time.
In the case of the laser focusing lens, however, the target material itself forms one end of the laser cavity and thus must initially be present at the proper location to cause lasing to occur. The lasing in turn causes the change in lens temperature for which compensation is desired. For this reason, changing the focal spot in this situation changes the end of the laser cavity, and thus also changes the power that goes into the lens. Since both the power and focal spot are changing simultaneously, determining the thermal effect on the lens and the focal spot becomes a much more complicated problem.
Because of this interaction, it has proven difficult to characterize the shift of the focal length, and thus the focal spot, of the lens under thermal load and to compensate for such shift. It is believed that existing EUV systems do not address this issue, and that the users thereof simply live with the decrease in efficiency that accompanies thermal loading and the resulting shift of the focal spot.